The present invention relates to a stepper motor driving apparatus and particularly to a stepper motor driving apparatus improved in a process of initializing a stepper motor used in an on-vehicle meter or the like.
For the reason of indication accuracy and cost, a stepper motor has been recently put into wide use in an on-vehicle meter such as a speed meter for indicating vehicle velocity or a tachometer for indicating the number of engine revolutions.
In a vehicle equipped with an on-vehicle meter using such a stepper motor, there is however a possibility that a difference between the required amount of movement of an indicating pointer interlocked with the rotation of the stepper motor and the actual amount of movement of the indicating pointer may be generated by a mistaken drive signal caused by vibration of the vehicle, noise, etc.
Therefore, in an on-vehicle meter using such a stepper motor, an initializing process is carried out so that the stepper motor is rotated backward in a stopper direction, for example, at switching-on timing of an ignition switch to restore the indicating pointer to a zero position decided by the stopper.
In the initializing process, whether the indicating pointer position-controlled by the stepper motor comes into contact with the stopper deciding the zero position of the indicating pointer is detected as follows. An induced voltage generated on the basis of change of magnetic flux in accordance with the rotation of a rotor of the stepper motor is detected. Zero position detection is performed so that when the detected induced voltage is not higher than a predetermined threshold, a decision is made that the indicating pointer has been stopped because of collision with the stopper disposed in the zero position.
For example, a stepper motor driving apparatus in which such zero position detection can be performed has been disclosed in International Patent Publication No. WO97/37425.
FIGS. 8 and 9 are block circuit diagrams of a related stepper motor driving apparatus disclosed in the WO97/37425. A stepper motor 10 includes four winding wires 1 to 4, and four switches 5 to 8 connected in series to the winding wires respectively. These winding wires 1 to 4 are connected between a positive supply potential UB, for example, of a car battery and a ground potential 0 by the switches 5 to 8 respectively. Tap terminals 11 to 14 are provided between the winding wires and the switches, respectively. The tap terminals 11 to 14 are used for monitoring voltages of the winding wires 1 to 4 respectively in order to specify a stopper and blocking. When the switches 5 to 8 are controlled to be opened or closed, the winding wires 1 to 4 included in the stepper motor 10 are connected to or disconnected from the supply potential UB. That is, the winding wires 1 to 4 are in a current conduction state or in a current non-conduction state.
In the stepper motor including the switches 5 to 8 disposed between the ground potential 0 and the winding wires 1 to 4 respectively as shown in FIG. 8, there is formed a stepper motor control portion having ground switches or low side drivers.
Next, in FIG. 9, an evaluation circuit 20 detects voltage peaks 51 and 52 of a voltage chart 50 induced in each current non-conduction winding wire as shown in FIG. 10. When the voltage peaks 51 and 52 are larger than a predetermined threshold, the evaluation circuit 20 specifies elastic blocking and generates a suitable output signal. The output signal serves as an external interrupt signal supplied to a micro-controller provided in a not-shown control circuit for controlling the stepper motor 10.
The voltage peaks 51 and 52 are different in polarity. This fact will be understood well when a zero line 53 shown in FIG. 10 is used as a reference line. The polarity can be measured appropriately by the evaluation circuit 20. More advantageously, detection can be executed even in the case where the stepper motor operates slowly because the stepper motor is out of order. The evaluation circuit need not have a high time sensitivity. Accordingly, inexpensive constituent elements can be used.
The evaluation circuit 20 shown in FIG. 9 has four branch paths which are the same in configuration and which correspond to the four winding tap terminals 11 to 14 shown in FIG. 8. Comparators 21 corresponding to the winding wires are provided in the branch paths respectively so as to be independent of one another. Each of the comparators 21 has a noninverting input terminal designated by “+”, and an inverting input terminal designated by “−”. The inverting input terminal of each comparator 21 is connected to a predetermined potential through a voltage divider which is composed of resistors 22 and 23 and which is connected between the supply potential UB and the ground potential 0. The noninverting input terminal of each comparator 21 is connected to a predetermined potential through a voltage divider which is composed of resistors 24 and 25 and which is connected between the supply potential UB and the ground potential 0. A series circuit composed of a diode 26 and a capacitor 27 is connected between each of the input terminals 11 to 14 of the evaluation circuit 20 and the noninverting input terminal of corresponding one of the comparators 21. The polarity of each diode 26 is decided as follows. That is, the polarity of each diode 26 is decided so that only a negative voltage, e.g. a voltage peak 52 shown in FIG. 10 can reach the noninverting input terminal “+” of a corresponding comparator 21 through a corresponding capacitor 27.
A threshold for the comparator 21 is decided in the inverting input terminal “−” by the voltage divider composed of the resistors 22 and 23. The voltage divider composed of the resistors 24 and 25 is provided so that only the voltage peak 52 exceeding a predetermined potential can reach the noninverting input terminal. The voltage in the inverting input terminal of the comparator 21 is decided by the voltage divider composed of the resistors 22 and 23 so that the voltage is lower than the voltage in the noninverting input terminal. Only the negative voltage is input and coupled to the noninverting input terminal of the comparator 21 by the diode 26 while only the edge of the negative voltage is input and coupled to the noninverting input terminal by the capacitor 27. The resistance ratio at a tap of the voltage divider composed of the resistors 22 and 23 is set to be equal to the resistance ratio at a tap of the voltage divider composed of the resistors 24 and 25. Accordingly, even in the case where the supply potential UB fluctuates, a signal output from the comparator 21 does not depend on the fluctuation of the supply potential UB because the two voltage dividers can lead the fluctuation of the supply potential UB in the same ratio.
The comparators 21 have output terminals 28 respectively. The output terminals 28 are led to an output terminal 200 common to all the comparators 21 through negatively polarized diodes 29 respectively. The common output terminal 200 is further connected to a reference voltage source Uref through a resistor 201 and connected to the ground potential 0 through a capacitor 202. As described above, when the voltage peak 52 of the voltage chart 50 induced in a current non-conduction winding wire exceeds a reference value, that is, when elastic blocking is specified, a signal is generated at the common output terminal 200. The output signal generated at the common output terminal 200 for designating blocking is supplied as an external interrupt signal to the micro-controller in the control circuit of the stepper motor. In the micro-controller, the output signal is further processed appropriately.
The fact that a reversed phase voltage is induced in a current non-conduction winding wire when an armature rotates backward is used in the evaluation for specifying elastic blocking. This voltage in the noninverting input terminal of at least one of the comparators 21 is lower than the voltage in the inverting input terminal. As a result, the output of the comparator 21 is switched. Accordingly, an interrupt is triggered so that blocking of the stepper motor is specified. On the contrary, when the stepper motor is operating, the voltage in the noninverting input terminal of the comparator 21 is kept higher than the voltage in the inverting input terminal.